In measuring an AC (alternating current) voltage of an input signal, the input AC voltage is converted to a corresponding DC (direct current) voltage, and then the DC voltage is measured by an AD (analog to digital) converter. An example of AC/DC converter in the conventional technology for measuring an AC voltage is shown in FIGS. 4 and 5. FIG. 4 is a block diagram showing a configuration of an AC/DC converter for measuring an AC voltage in the conventional technology. FIG. 5 shows a waveform diagram in a calibration process of the AC/DC converter of FIG. 4.
The AC/DC converter of FIG. 4 includes a signal generator 11 for generating a step voltage signal, a switch 31 for changing between an input signal and the step voltage signal from the signal generator 11, an amplifier 20 for amplifying an incoming signal, a switch 32, a sample hold 30 for sampling the incoming signal and holding the sampled voltage for a predetermined period of time, an RMS/DC converter 40 for converting an effective (root-mean-square) value of an AC voltage to a DC voltage, a switch 33 for changing between the outputs of the sample hold 30 and the RMS/DC converter 40, and an A/D converter 50. The amplifier 20 has a circuit arrangement such as shown in FIG. 3 to be able to adjust its frequency response curve.
To measure the AC voltage of the input signal, the switch 31 is set to a terminal (a) and the switches 32 and 33 are set to terminals (b), respectively. The AC voltage to be measured is amplified by the amplifier 20 and is converted to a DC voltage by the RMS/DC converter 40. The DC voltage is converted to digital data representing the AC voltage by the A/D converter 50.
Since an AC waveform of the input signal to be measured includes a wide range of frequency components, the AC/DC converter needs to have a flat frequency (frequency response curve ) characteristic in a wide range of frequency. To satisfy this requirement, in advance to the actual measurement of the input AC voltage, a calibration operation is performed in the AC/DC converter.
In the calibration operation, for correcting the frequency response of the amplifier 20, the switch 31 is set to the terminal (b) and the switches 32 and 33 are set to the terminals (a), respectively. The step signal from the signal generator 11 is supplied to the amplifier 20. As shown in FIG. 5, the step voltage signal 100 must have an accurate step waveform. The output of the amplifier 20 is provided to the sample hold 30. The sample hold 30 must have a high speed and high resolution capability to fully detect the transient response of the step signal at the output of the amplifier 20.
The sample hold 30 samples and holds the output of the amplifier 20. The voltage held by the sample hold 30 is read by the A/D converter 50 in digital data. The resulted data show different waveforms depending upon the frequency response characteristics of the amplifier 20. For example, the waveforms 101-103 in FIG. 5(a) may be produced at the output of the amplifier. The waveforms 101 and 103 show that the frequency characteristic of the amplifier 20 is inadequate since the waveform does not accurately reproduce the input step signal. In contrast, the waveform 102 shows that the frequency response of the amplifier is satisfactory for the intended frequency range.
Thus, by adjusting the frequency response curve of the amplifier 20, the frequency response of the amplifier 20 is adjusted to produce the accurate waveform such as the waveform 102 of FIG. 5(a). When the amplifier 20 is adjusted to output the step waveform 102 of FIG. 5(a), the frequency response curve is considered to be flat throughout the measuring frequency range.
Alternatively, the following procedure may be taken for adjusting the frequency response characteristic of the amplifier 20. While applying an input step signal 100 as shown in FIG. 5(a), the output of the amplifier 20 is measured two times through the sample hold 30 and the A/D converter 50 in the manner noted above. One measurement is made at the timing immediately after the step transition of the step voltage signal to determined the transient response of the amplifier 20. The other measurement is made at the timing sufficiently after the transient period, i.e., for the steady state of the input step signal.
The two measurement results are compared and the frequency response characteristic of the amplifier 20 is adjusted so that the two measurement results show the same value. When the amplifier 20 is adjusted to output the same measurement results between the first and second measurements for the step signal 100 of FIG. 5(a), the frequency response curve is considered to be flat throughout the measuring frequency range.
In the above noted calibration procedures, the step signal to be supplied to the amplifier must have an ideal step waveform. Therefore, to generate such an ideal step signal, the signal generator 11 is required to have a high performance capability to generate the calibration signal with high precision.
An example of a circuit arrangement for adjusting the frequency response curve in the amplifier 20 is shown in FIG. 3. The amplifier 20 has an attenuator 21 having an AC ladder configuration, an adjustable resistor Ra, a buffer amplifier 22, a variable gain amplifier 23, and a D/A converter 24. The details of the amplifier having the frequency characteristic correction ability is shown, for example, in Japanese Utility Model application No. 1993-4342.
To adjust the frequency response of the attenuator 21 of an AC ladder circuit, either capacitance or resistance of the ladder circuit may be regulated. In the amplifier 20 of FIG. 3, a combination of the adjustable resistor Ra and the variable gain amplifier 23 functions as a variable resistor and varies the overall frequency characteristic of the amplifier by changing the gain of the variable gain amplifier. The D/A converter 24 provides a control voltage to the variable gain amplifier 23. Thus, an automatic adjustment of the frequency response may be possible by providing digital data to the D/A converter based on the measurement results.
In the above noted calibration procedures, the step signal to be supplied to the amplifier must have an ideal step waveform. Namely, the waveform of the step signal must include sufficiently wide range of frequency components represented by its rising edge. The waveform of the step signal must not show an over shoot or an under shoot during the process of transition. The steady state voltage must be an accurate DC voltage.
Therefore, to generate such an ideal step signal, the signal generator 11 is required to have a high level capability such as a high speed and high precision. Further, to accurately sample the transient waveform of the step voltage signal, the sample hold 30 needs to operate with high speed and high resolution. It is difficult or practically inconvenient to realize such a high performance signal generator and a sample hold circuit.